1. Field of the Invention
The present invention relates to a probe for electronic scanning type ultrasonic diagnostic apparatus.
2. Description of the Prior Art
In ordinary ultrasonic diagnostic apparatus, the electro-acoustic transducers comprise piezoelectric resonators which direct an ultrasonic pulse at the portion of a living body to be observed, and successively detect the ultrasonic pulse as it is reflected from the boundaries of the organs of the living body. By changing the direction of the ultrasonic pulse which is directed into the living body, two dimensional information concerning the structure of the organs of the living body is obtained and displayed on a CRT.
In order to easily obtain two dimensional information concerning the structure of each portion of the living body, the probe which houses the plurality of electro-acoustic transducers is normally separated from the diagnostic apparatus and is connected to the apparatus by an electric cable.
As described above, in such an ultrasonic diagnostic apparatus for scanning by ultrasonic waves, it is necessary to change successively both the transmission direction of ultrasonic pulses directed into the living body and the reception direction of reflected ultrasonic signals from the living body. Although such a scanning operation may be carried out by mechanically changing the orientation of a transducer, it may also be carried out by sequentially driving a plurality of transducers or by successively shifting a drive timing thereof. The ultrasonic diagnostic apparatus in which ultrasonic scanning is carried out by electronically controlling a plurality of transducers in this manner is called an electronic scanning type ultrasonic diagnostic apparatus.
There are two scanning types of electronic scanning type ultrasonic diagnostic apparatus. One is the linear scanning type and the other is the sector scanning type. In the linear scanning type, scanning is executed by changing over ultrasonic transducers by means of an electronic switch for transmission and reception of the ultrasonic wves, and in the sector scanning type, scanning is executed by changing an equiphase plane by means of a relative change of transmission and reception timings of the ultrasonic waves. In both of these types of apparatus, the ultrasonic probe in which a plurality of ultrasonic transducers are arranged in a row is essential. In a probe having such a structure, however, the ultrasonic directivities of the transducers which are near to the end of the row differ from those of the transducers which are near to the center of the row. This has an adverse effect on the ultrasonic scanning and gives rise to the problem that with an ultrasonic diagnostic apparatus using such an probe a high quality sectional image is not obtained.
This problem will now be described in detail. FIG. 1 shows a cross sectional view of the conventional probe. On a supporting plate 1 which is an ultrasonic wave absorber, a plurality of ultrasonic transducers 2-1 to 2-N are arranged in a row and fixed by backing material 3. If the thickness oscillation of the transducers is to be utilized, each transducer is provided with electrodes 4a and 4b respectively on the upper and lower faces thereof. In these electrodes, each electrode 4a provided on the upper face of the transducer, is grounded, while high voltage pulses are applied to the electrode 4b, or electric signals corresponding to the reflected ultrasonic waves are extracted therefrom. Surfaces of the electrodes 4a are covered with a coating layer (not shown), for example with epoxy resin, in order to protect the electro-acoustic transducers 2-1 to 2-N and to obtain good and comfortable contact to a living body. In addition, spacer 5 made of an ultrasonic absorber may be inserted in between these transducers. In this way, the mutual influence between these transducers 2-1 to 2-N may be minimized. However, in fact, oscillations of each of the transducers 2-1 to 2-N still do affect oscillations of neighbouring transducers, due to the supporting plate 1, backing material 3, spacer 5 and coating layer etc. This influence is felt both in terms of mechanical interference and electrical impedence matching. While the structure of the device is made to neutralize these effects, still some influence remains. Consequently, when each of the transducers 2-1 to 2-N oscillates, it interferes with the other transducers.
Of the transducers which are arranged in a row, those transducers which are near to the center have generally similar ultrasonic directivities since they are arranged with approximately the same configuration and environment one each side thereof. Therefore, each of the transducers which is near to the center shows a symmetrical directivity. However, those transducers which are near to the end of the row, especially the transducers 2-1 and 2-N which are provided at the left and right ends, while having similar transducers on one side, have no transducer on the other side. These transducers therefore show ultrasonic directivities which are different from those of the transducers which are near to the center. Namely, each of the transducers 2-1 and 2-N shows an asymmetrical directivity. Such asymmetrical directivities of the transducers which are near to the end of the row exert a harmful influence as a whole upon the directivity characteristics for transmission and reception of ultrasonic waves. As a result, a sectional image of high quality can not be obtained in the case where the aforementioned probe is used in the ultrasonic diagnostic apparatus.
On the other hand, generally in the sector type ultrasonic diagnostic apparatus, all transducers are simultaneously used and the scanning is carried out by changing the drive timing of the transducers. Therefore the directivity characteristic of each transducer is desirably, as far as possible, non-directional. However, when the conventional probe in which the transducers arranged in a row is used, the directivity characteristics of the transducers which are near to the end of the row differ from those of the transducers which are near to the center, so the adverse effect on the image, described above, is particularly distinguished in the case of such a sector scanning type ultrasonic diagnostic apparatus.
In the probe in which a plurality of transducers are arranged in a row, it is known that an acoustic lens having a focusing effect can be provided on the transducers in order to sharpen the directivity characteristics at the plane intersecting the direction of the row. Such a construction is described in detail in U.S. Pat. No. 3,936,791. The probe having the acoustic lens is advantageous only for sharpening the directivity characteristics at the intersecting plane. However, even though a probe of this type is used, deterioration of image quality still appears, because the directivity characteristics of the transducers which are near to the end of the row are still asymmetrical.